Description

Return the 'estimated' extent of the given spatial table.
The estimated is taken from the geometry column's statistics. The
current schema will be used if not specified. The default behavior
is to also use statistics collected from child tables (tables
with INHERITS) if available. If 'parent_only' is set to TRUE, only
statistics for the given table are used and child tables are
ignored.

For PostgreSQL>=8.0.0 statistics are gathered by VACUUM
ANALYZE and resulting extent will be about 95% of the real
one.

In absence of statistics (empty table or no ANALYZE called) this function
returns NULL. Prior to version 1.5.4 an exception was thrown
instead.

For PostgreSQL<8.0.0 statistics are gathered by
update_geometry_stats() and resulting extent will be exact.